Ohio is set to enact a law that allows teachers and other staff to be armed with guns in schools once they have completed up to 24 hours of initial training.
Proponents hope armed teachers will reduce the frequency and deadliness of school shootings, which have become recurrent in the United States.
The bill's opponents, including teachers' unions and the state's main police union, say it will only make schools more dangerous for children.
The bill was finalised 10 days after a teenager with an AR-15-style rifle killed 19 students and two teachers in Texas.
Ohio Governor Mike DeWine, a Republican, has said he will sign the bill into law.
Proponents of the bill said it would allow school staff to confront an armed attacker before police entered.
"In emergency situations at our schools, seconds matter and tragedies can be prevented," Representative Thomas Hall, the bill's sponsor, said in a statement.
Armed teachers would be required to undergo criminal background checks and receive eight hours of additional training each subsequent year.
Under the new law, school districts would have to notify parents if they decide to let armed teachers onto school premises.
Congressman withdraws following support for gun control
Meanwhile, a Republican congressman who came out in support of gun control after a mass shooting in his area dropped out of the race for his re-election on Friday upon coming under withering criticism from Republicans who saw his policy shift as a betrayal.
Chris Jacobs, a first-term US representative from suburban Buffalo, New York, said he decided to withdraw to avoid "an incredibly divisive election" for the Republican Party.
Mr Jacobs embraced a federal ban on assault weapons and other gun control measures a week ago in the wake of two massacres.
A white gunman killed 10 black people inside a Buffalo supermarket on May 14 in a racially motivated attack, authorities said.
"This has been a profoundly impactful event for me," Mr Jacobs told the Buffalo News, referring to the shooting.
The backlash was immediate. Gun rights groups posted his office and mobile numbers on the internet and local party leaders began pulling their support, the New York Times reported.
"The last thing we need is an incredibly negative, half-truth-filled media attack funded by millions of dollars of special interest money coming into our community around this issue of guns and gun violence and gun control," Mr Jacobs said after announcing his withdrawal.
National Gun Violence Awareness Day
Friday also marked National Gun Violence Awareness Day in the US, with landmarks turning orange and marches demonstrating popped up around the country.
Children led a sea of demonstrators wearing orange gun violence awareness movement T-shirts.
They chanted: "Don't shoot, I wanna grow up!" across the Brooklyn Bridge in New York.
"It doesn't start and end with a bullet. It's seen, it's heard, it's felt," said Ian Stuart, a coordinator at the Center for Court Innovation.
"We have to start changing the conversation to really challenge these norms that allow for violence to be a means of communication because it's not. Violence is what happens when communication breaks down."
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